1. Field of the Invention
This invention relates to stabilizers of ammonium dinitramide (ADN). More particularly, the ADN stabilizers are heterocyclic organic compounds added to ADN for use as an oxidizer for propellents, pyrotechnics, gas generators, explosives and like formulations. Most particularly, the stabilizer is a substituted pyridine, pyrimidine, pyrazine or triazine, and/or derivatives thereof. The stabilizers increase the thermal stability and the shelf or service life of the ammonium dinitramide, and increase the reliability of the formulations incorporating ADN over extended periods of time and/or after exposure to temperature changes.
2. Description of Related Art
Ammonium perchlorate (AP) is well known as an oxidizer for composite solid propellents. However, AP emits hydrogen chloride in the exhaust gas, impeding its use in several types of applications and raising objections to its use because of environmental degradation. One possible alternative to AP is ADN which is higher in performance, but does not produce hydrogen chloride as a decomposition product.
ADN is useful as an oxidizer for highly energetic materials, such as propellents, pyrotechnics, and gas generator formulations for such uses as airbag deployment, solid rocket motors, explosives, and the like. ADN is a compound comprising nitrogen, hydrogen and oxygen that can provide a clean exhaust gas, one composed of invisible, nontoxic gases such as nitrogen, carbon dioxide and water vapor. For this reason, in a tactical military scenario, ADN produces a reduced smoke exhaust compared to AP, allowing better protection from discovery for the launch site, as well as providing a more environmentally benign exhaust. The clean exhaust also results in greater occupational safety for crews in confined launch areas, as well as less missile signature during flight.
ADN, as manufactured, is difficult to formulate since it forms crystals of excessive length, making it process poorly in propellent formulations, raising their viscosity and making them difficult to cast. The cooling of molten droplets to form tiny spheres, or prilling, may be used to manufacture the ADN in a more suitable form for processing, but prilling involves melting the ADN, and then stirring or spraying, which necessitates a stabilizer to retard thermal decomposition. The manufacture of ADN has been disclosed in U.S. Pat. No. 5,659,080 (Suzuki et al.) and U.S. Pat. No. 5,714,714 (Stem et al.), the disclosure of these patents are herein incorporated by reference. However, ADN containing compositions tend to decompose when aged at temperatures above ambient. ADN decomposes into nitrous acid (HNO.sub.2), nitric acid (HNO.sub.2 O), nitrous oxide (N.sub.2 O), , nitrogen dioxide (NO.sub.2), ammonium nitrate (NH.sub.4 NO.sub.3) and water (H.sub.2 O). The presence of these decomposition products increases the rate of decomposition of the remaining ADN.
Hexamethylenetetramine (hexamine) has been used to stabilize ADN. Generally, hexamethylenetetramine is added prior to prilling. The amount of hexamethylenetetramine used typically is 0.3 to 0.6 weight percent. Addition of 0.3-0.6% of hexamine to ADN melts kept the gas evolution from thermal decomposition low for as long as 6 hours at 120.degree. C. However, when hexamethylenetetramine has fully reacted with the decomposition products of ADN, it no longer is able to inhibit the decomposition of the ADN. Additional decomposition of the ADN after this point results in a vigorous oxidation of any proximate fuel and binder in a formulation, as well as the partially oxidized hexamine, in the ADN. This causes instability and degraded performance and/or safety characteristics in the formulation, either in storage or use.
Pyridine and pyridone have been described as stabilizers for hydroxylammonium nitrate (HAN) and hydroxylamine in U.S. Pat. No. 5,703,323 (Rothgery et al.). However, Rothgery et al. uses pyridine and pyridone as chelation reagents for iron impurities, which catalyze decomposition of HAN and hydroxyl amine. Rothgery et al. mentions the effect of the decomposition products of the hydroxylammonium nitrate and hydroxylamine on the stability of the remaining material, but does not address the scavenging, absorption or neutralization of these decomposition products.
Stable charge-transfer complexes can be formed with acids by nitrogen-containing heterocyclic compounds. In the past, such oxidizers as hydrazinium diperchlorate have been stabilized by addition of these compounds, which include riboflavin and 7-(2,3-dihydroxypropyl)theophylline. Free-radical scavengers, which can also react with acids and nitrogen oxides have been used as stabilizers for hydroxylammonium perchlorate. These include triallyl-1,3,5-triazine-2,4,6-(1H,2H,5H)-trione and triallyloxy-1,3,5-triazene.
Ideally, stabilizers for oxidizers such as ADN should be effective in the smallest possible quantities. Addition of non-energetic compounds to high-energy compounds reduces the energy available to the formulation. Additionally, the incorporation of organic materials into oxidizers may cause sensitivity problems. There is a need for ADN stabilizers having minimal weight and volume relative to their stabilizing effect on the ADN. These ADN and stabilizer compositions should be useful for increasing storage time and resistance to elevated temperatures, resulting in safer handling and aging characteristics particularly when used in propellant formulations for tactical missiles. The present invention addresses these needs.